Link State Routing K. Talaulikar
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc.
Expires: February 21, 2020 H. Johnston
AT&T Labs
August 20, 2019
OSPF Reverse Metricdraft-ketant-lsr-ospf-reverse-metric-02
Abstract
This document specifies the extensions to OSPF that enables a router
to signal to its neighbor the metric that the neighbor should use
towards itself using link-local advertisement between them. The
signalling of this reverse metric, to be used on link(s) towards
itself, allows a router to influence the amount of traffic flowing
towards itself and in certain use-cases enables routers to maintain
symmetric metric on both sides of a link between them.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 21, 2020.
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This document describes certain use-cases where it is desirable for a
router to be able to signal what we call as the "reverse metric" (RM)
to its neighbor to adjust the routing metric on the inbound
direction. When R1 signals its reverse metric on its link to R2,
then R2 advertises this value as its metric to R1 in its Router-LSA
instead of its locally configured value. Once this information is
part of the topology then all other routers do their computation
using this value which results in the desired change in traffic
distribution that R1 wanted to achieve towards itself over the link
from R2.
This document proposes an extension to OSPF link-local signaling
(LLS) [RFC5613] for signalling the OSPF reverse metric using the LLS
Reverse Metric TLV in Section 4, the reverse Traffic Engineering (TE)
metric [RFC3630] using the LLS Reverse TE Metric TLV in Section 5 and
describes the related procedures in section Section 6.
2. Use Cases
This section describes certain use-cases that OSPF reverse metric
helps to address. The usage of OSPF reverse metric need not be
limited to these cases and is intended to be a generic mechanism.
2.1. Symmetrical Metric Based on Reference Bandwidth
Certain OSPF implementations and deployments deduce the metric of
links based on their bandwidth using a reference bandwidth. The OSPF
MIB [RFC4750] has ospfReferenceBandwidth that is used by entries in
the ospfIfMetricTable. This mechanism is leveraged in deployments
where the link metrics get lowered or increased as bandwidth capacity
is removed or added e.g. consider layer-2 links bundled as a layer-3
interface on which OSPF is enabled. In the situations where these
layer-2 links are directly connected to the two routers, the link and
bandwidth availability is detected and updated on both sides. This
allows for schemes where the metric is maintained to be symmetric in
both directions based on the bandwidth.
Now consider variation of the same deployment where the links between
routers are not directly connected and instead are provisioned over a
layer-2 network consisting of switches or other mechanisms for a
layer-2 emulation. In such scenarios, as show in Figure 1, the
router on one side of the link would not detect when the neighboring
router has lost one of its layer-2 link and has reduced capacity to
its layer-2 switch. Note that the number of links and their
capacities on the router R0 may not be the same as those on R1, R2
and R3. The left hand side diagram shows the actual physical
topology in terms of the layer-2 links while the right hand side
diagram shows the logical layer-3 link topology between the routers.
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towards the core and it needs to reduce the traffic going through it
and perhaps redirect some of that load via AGGR2 which is not facing
a similar issue. Altering its own metric towards Rx routers would
influence the traffic flowing through it in the direction from core
to the Rx but not the other way around as desired.
Core Network
^ ^
| |
V v
+----------+ +----------+
| AGGR1 | | AGGR2 |
+----------+ +----------+
^ ^ ^ ^
| | | |
| +-----------+ |
| | | |
| +--------+ | |
v v v v
+-----------+ +-----------+
| R1 | | RN |
| Router | ... | Router |
+-----------+ +-----------+
Figure 2: Adaptive Metric for Dual Gateways
In such a scenario, the AGGR1 router could signal an incremental
value of OSPF reverse metric towards some or all of the Rx routers.
When the Rx routers apply this signaled reverse metric offset value
to the original metric on their links towards AGGR1 then the path via
AGGR2 becomes a better path causing traffic towards the core getting
diverted away from it. Note that the reverse metric mechanism allows
such adaptive metric changes to be applied on the AGGR1 as opposed to
being provisioning statically on the possibly large number of Rx
routers.
3. Solution
To address the use-cases described earlier and to allow an OSPF
router to indicate its reverse metric for a specific point-to-point
or point-to-multipoint link to its neighbor, this document proposes
to extend OSPF link-local signaling to advertise the Reverse Metric
TLV in OSPF Hello packets. This ensures that the RM signaling is
scoped ONLY to each specific link individually. The router continues
to include the Reverse Metric TLV in its Hello packets on the link as
long as it needs its neighbor to use that metric value towards
itself. Further details of the procedures involve are specified in
Section 6.
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The RM signaling specified in this document is not required for
broadcast or non-broadcast-multi-access (NBMA) links since the same
objective is achieved there using the OSPF Two-Part Metric mechanism
[RFC8042].
4. LLS Reverse Metric TLV
The Reverse Metric TLV is a new LLS TLV. It has following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTID | Flags |O|H| Reverse Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD, suggested value 19
Length: 4 octet
MTID : the multi-topology identifier of the link ([RFC4915])
Flags: 1 octet, following are defined currently and the rest MUST
be set to 0 and ignored on reception.
* H (0x1) : Indicates that neighbor should use value only if
higher than its current metric value in use
* O (0x2) : Indicates that the reverse metric value provided is
an offset that is to be added to the original metric
Reverse Metric: 2 octets, the value or offset of reverse metric to
be used
5. LLS Reverse TE Metric TLV
The Reverse TE Metric TLV is a new LLS TLV. It has following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |O|H| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse TE Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD, suggested value 20
Length: 4 octet
Flags: 1 octet, following are defined currently and the rest MUST
be set to 0 and ignored on reception.
* H (0x1) : Indicates that neighbor should use value only if
higher than its current TE metric value in use
* O (0x2) : Indicates that the reverse TE metric value provided
is an offset that is to be added to the original TE metric
RESERVED: 24-bit field. SHOULD be set to 0 on transmission and
MUST be ignored on receipt.
Reverse TE Metric: 4 octets, the value or offset of reverse
traffic engineering metric to be used
6. Procedures
When a router needs to signal a RM value that its neigbhor(s) should
use towards itself, it includes the Reverse Metric TLV in the LLS
block of its hello messages sent on the link and continues to include
this TLV for as long as it needs it's neighbor to use this value.
The mechanisms used to determine the value to be used for the RM is
specific to the implementation and use-case and is outside the scope
of this document. e.g. in the use-case related to symmetric metric
described in Section 2.1, the RM value may be derived based on the
router's link's bandwidth with respect to the reference bandwidth.
A router receiving a hello packet from its neighbor that contains the
Reverse Metric TLV on its link SHOULD use the RM value to derive the
metric for the link in its Router-LSA to the advertising router.
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When the O flag is set, the value in the TLV needs to be added to the
existing original metric provisioned on the link to derive the new
metric value to be used. When the O flag is clear, the value in the
TLV should be directly used as the metric to be used. When H flag is
set and O flag is clear, this is done only when the RM value signaled
is higher than the provisioned metric value being used already. This
mechanism applies only for point-to-point, point-to-multipoint and
hybrid broadcast point-to-multipoint ( [RFC6845]) links. For
broadcast and NBMA links the OSPF Two-Part Metric mechanism [RFC8042]
should be used in similar use-cases.
Implementations SHOULD provide a configuration option to enable the
signaling of RM from a router to its neighbors and MAY provide a
configuration option to disable the acceptance of the RM from its
neighbors.
A router stops including the Reverse Metric TLV in its hello messages
when it needs its neighbors to go back to using their own provisioned
metric values. When that happens, a router which had modified its
metric in response to receiving a Reverse Metric TLV from its
neighbor should revert back to using its original provisioned metric
value.
In certain scenarios, it is possible that two or more routers start
the RM signaling on the same link. This could create collision
scenarios. The following rules MUST be adopted by routers to ensure
that there is no instability in the network due to churn in their
metric due to signaling of RM:
o The RM value that is signaled by a router to its neighbor MUST NOT
be derived from the reverse metric being signaled by any of its
neighbor on any of its links.
o The RM value that is signaled by a router MUST NOT be derived from
its own metric which has been modified on account of a RM signaled
from any of its neighbors on any of its links. RM signaling from
other routers can affect the router's own metric advertised in its
Router-LSA. When deriving the RM values that a router signals to
its neighbors, it should use its "original" local metric values
not influenced by any RM signaling.
Based on these rules, a router MUST never start or stop or change its
RM metric signaling based on the RM metric signaling initiated by
some other router. Based on the local configuration policy, each
router would end up accepting the RM value signaled by its neighbor
and there would be no churn of metrics on the link or the network on
account of RM signaling.
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In certain use-case as described in Section 2.1 when symmetrical
metrics are desired, the RM signaling can be enabled on routers on
either ends of a link. In other use-cases as described in
Section 2.2 RM signaling may need to be enabled on only router at one
end of a link.
When using multi-topology routing with OSPF [RFC4915] a router MAY
include multiple instances of the Reverse Metric TLV in the LLS block
of its hello message - one for each of the topology for which it
desires to signal the reserve metric for.
In certain scenarios, the OSPF router may also require the
modification of the TE metric being advertised by its neighbor router
towards itself in the inbound direction. The Reverse TE Metric TLV,
using similar procedures as described above, MAY be used to signal
the reverse TE metric by a router. The neighbor SHOULD use the
reverse TE metric value to derive the TE metric to be used in the TE
Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630].
7. Backward Compatibility
The signaling specified in this document happens at a link-local
level between routers on that link. A router which does not support
this specification would ignore the Reverse Metric and Reverse TE
Metric LLS TLVs and take no actions to updates its metric in the
other LSAs. As a result, the behavior would be the same as before
this specification. Therefore, there are no backward compatibility
related issues or considerations that need to be taken care of when
implementing this specification.
8. IANA Considerations
This specification updates Link Local Signalling TLV Identifiers
registry.
Following values are requested for allocation:
o TBD (Suggested value 19) - Reverse Metric TLV
o TBD (Suggested value 20) - Reverse TE Metric TLV
9. Security Considerations
The security considerations for "OSPF Link-Local Signaling" [RFC5613]
also apply to the extension described in this document. The usage of
the reverse metric TLVs is to alter the metrics used by routers on
the link and influence the flow and routing of traffic over the
network. Hence, modification of the Reverse Metric and Reverse TE
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